www.j-sens-sens-syst.net/5/73/2016/ doi:10.5194/jsss-5-73-2016
© Author(s) 2016. CC Attribution 3.0 License.
Joining technologies for a temperature-stable integration
of a LTCC-based pressure sensor
J. Schilm1, A. Goldberg1, U. Partsch1, W. Dürfeld2, D. Arndt2, A. Pönicke3, and A. Michaelis1 1Fraunhofer IKTS, Fraunhofer Institute for Ceramic Technologies and Systems, Winterbergstraße 28,
01277 Dresden, Germany
2ALL IMPEX GmbH, Bergener Ring 43, 01458 Ottendorf-Okrilla, Germany 3Modine Europe GmbH, Arthur-B.-Modine-Straße 1, 70794 Filderstadt, Germany
Correspondence to:J. Schilm (jochen.schilm@ikts.fraunhofer.de)
Received: 26 October 2015 – Revised: 13 January 2016 – Accepted: 26 January 2016 – Published: 9 March 2016
Abstract. Besides the well-known application as circuit boards and housings, multilayer low-temperature co-fired ceramics (LTCC) offer a flexible and temperature-stable platform for the development of complex sensor elements. Commercial LTCC qualities are usually available with a matching set of metallization pastes which allow the integration of various electrical functions. However, for the integration of ceramic sensor elements based on LTCC into standardized steel housings it is necessary to compensate the mismatching thermal expan-sion behaviour. Therefore balancing elements made of Kovar®(Fe–29 wt% Ni–17 wt% Co) and alumina ceramic (Al2O3) can be used. These components have to be joined hermetically to each other and to the LTCC sensors.
In this study, brazing experiments were performed for combinations of Kovar–Al2O3 and Kovar–LTCC with
Ag–Cu–Ti- and Ag–Cu–In–Ti-based commercial braze filler metals, Cusil-ABA®and Incusil®-ABA, respec-tively. For both active braze filler metals, optimized processing parameters were investigated to realize hermetic Kovar–Al2O3and Kovar–LTCC joints.
1 Introduction
Low temperature co-fired ceramics (LTCC) is a well-known technology for highly integrated, reliable, and high-temperature-stable microelectronic packages in mobile com-munication or for automotive, space, or medical applications (Peterson et al., 2008). Due to its linear stress/strain be-haviour and its ability for integration of three-dimensional shapes like diaphragms, channels, and cavities, according to various authors, LTCC complies with all requirements for the integration of mechanical structures, e.g. for pressure sen-sors (Zarnik et al., 2010; Partsch et al., 2012; Fournier et al., 2010). A new piezo-resistive pressure sensor concept was de-veloped by Partsch et al. (2007). Figure 1 shows a overview of a completely assembled LTCC-based pressure sensor with housing, pressure port and wiring. This new design princi-ple allows the sensor cell to be fully mechanically decouprinci-pled and stress-free inside the sensor frame. Only thin LTCC can-tilevers containing microchannels are used for the pressure connection of the sensor cell. Thick film resistors,
Figure 1.LTCC pressure transmitter and corresponding drawing of inner setup.
calibration, and subsequent application, the LTCC sensor ele-ments have to be connected gas-tight to the measuring equip-ment. In most cases standardized steel connectors are used to ensure a gas-tight connection of the LTCC sensors to the sys-tem. But, the integration of LTCC-based sensor elements for high-temperature applications requires suitable interconnect technologies. For example, if the LTCC sensors and the steel connectors are glued directly together, the maximum opera-tion temperature of the sensor is limited to the glass transi-tion temperature of the epoxy resin, which is in most cases below 190◦C. Also soldering offers no reliable option as the joint strength decreases rapidly at higher temperatures due to interdiffusion processes, which in turn results in a loss of gas-tightness. For an increased thermal stability other join-ing technologies like glass sealjoin-ing or brazjoin-ing can be used. However, in this case the mismatching coefficients of ther-mal expansion of LTCC and steel will limit the lifetime of the integrated sensor elements as thermal cycles will initi-ate cracks along the sealing or inside the sensor. To over-come these problems, a stepwise integration of the LTCC-based sensor elements into steel connectors was developed. This approach offers the opportunity to outbalance the dif-ferent thermal expansion coefficients of LTCC and steel, and to increase the operation range of the sensor assembly to high temperatures up to 300◦C. The integration concept is schematically shown in Fig. 2. The integration approach of LTCC-based sensors can be divided into three steps. In step A a balancing element made of a nickel–cobalt ferrous alloy, i.e. Kovar®(Fe–29 wt% Ni–17 wt% Co) with a coefficient of thermal expansion that closely matches that of the ceramic materials at low temperatures, is brazed to the steel hous-ing. Such a steel connection produced from cost-efficient and construction steel types are necessary for having a standard interconnection interface which can be processed easily by
Figure 2.Single LTCC pressure and steel connect(a)and scheme for the stepwise integration of a LTCC-based sensor element into the steel connect(b).
electron beam welding to other devices or which may be a more complex device itself. The following step B is required in order to create a joinable surface for step C and will be described later in detail. In step B the bonding of a ceramic layer made from alumina (Al2O3) or LTCC offers the
pos-sibility to integrate the LTCC-based sensor elements in step C by sealing with a glass-based solder or other techniques. A convenient way for the implementation of step B is the so-called active metal brazing process.
1.1 Joining of ceramics to metals by brazing (step B)
Fernie et al. (2009) describes several direct bonding tech-niques to join ceramics and metals together for hermetic joints. Besides techniques without any liquid phase like dif-fusion bonding or friction welding, direct bonding methods utilizing a liquid phases based on adhesives, braze filler met-als, or glass solders can be used. For the desired application, where in joining step B (Fig. 2) a planar joint between Kovar and Al2O3or LTCC has to be realized which is suitable for
operation temperatures up to 300◦C, brazing is the appropri-ate method. According to Nascimento et al. (2003) a com-mon way to braze ceramics and metals is the metallization of the ceramic prior to the brazing process since then the met-allized ceramics can be brazed to metals without any active braze filler metal. However, the metallization processes im-ply several individual process steps, which makes them com-plicated and expensive. In contrast to this quite old technol-ogy Walker and Hodges (2008) describe active metal braz-ing as a technique which allows brazbraz-ing ceramics directly to metals or themselves without any additional metallization steps. Active brazing alloys are based on filler metals like Ag, Ag–Cu, or Au and contain low fractions of so-called ac-tive species (i.e. Ti, Zr, Hf). These elements enhance wet-ting of the ceramic surface during brazing in an oxygen-free environment using protective atmospheres or vacuum (p<10−4mbar).
In the literature active metal brazing of Kovar to Al2O3
is much more extensively investigated than the brazing of Kovar to LTCC. A good overview of the problem can be found in Walker and Hodges (2013). When Al2O3
braz-ing alloy. This reaction layer consists of a Ti-rich oxide layer with a thickness below 1 µm completely covering the alu-mina interface and a second, a few-microns-thick mixed ox-ide layer containing Ti, Cu, and Al (Stephens et al., 2003; Lin et al., 2014). Al2O3–Al2O3joints prepared in this
man-ner are hermetic and reach joint strengths of > 95 % of the strength of the base material. If now one of the Al2O3pieces
is substituted by Kovar, the results change. Hahn et al. (1998) demonstrated in their work that Kovar–Al2O3joints achieved
no high strength values and reached only 40 % of the ini-tial Al2O3–Al2O3joint strength. Additionally, Kovar–Al2O3
joints made by active metal brazing showed poor hermetic-ity. Stephens et al. (2000) revealed by microstructural analy-sis that in non-hermetic joints no continuous TixOyreaction layers were formed at the interface between Al2O3and the
brazing alloy. According to Vianco et al. (2003a) this phe-nomenon of titanium scavenging can be explained by the dis-solution of nickel and iron from Kovar in the molten braze and the strong affinity of nickel to titanium. Arróyave and Eagar (2003) describe that during the brazing process nickel and titanium react to form intermetallic compounds while the activity of titanium in the melt is decreased and the formation of the necessary reaction zone at the Al2O3interface is
sup-pressed. To improve the joint strength of Kovar–Al2O3and to
prevent the formation of intermetallic compounds, different barrier layer concepts were developed. Mo, Ni, and Mo–Ni coatings on Kovar were tested by Hahn et al. (1998). They re-port an increased bending strength of the joints by more than 80 % of the uncoated base material. Vianco et al. (2003b) in-vestigated the influence of Mo thickness and found that braz-ing of Kovar–Al2O3 with a 500 µm thick Mo barrier layer
yielded the best hermeticity performance and strength. Mag-netron sputtering of titanium layers on Al2O3was introduced
by Zhu et al. (2014) as an alternative method to improve the joint strength and gas-tightness of Kovar–Al2O3 joints.
The mechanical metallization of alumina surfaces was ap-plied by Nascimento et al. (2007) to achieve a proper pre-metallization of the ceramic component. Besides introducing barrier layers, Wielage et al. (2012) showed that an improve-ment of the joints is possible if induction brazing with much shorter brazing times compared to conventional furnace braz-ing is applied. Microstructural analysis showed a remarkable reduction of intermetallic compounds and a decrease of the reaction layer thickness between Kovar and the brazing alloy. For the desired application and workflow in step B (Fig. 2) it seems to be interesting to use LTCC as a joining partner for Kovar instead of Al2O3. The main reason for the use
of LTCC is given by minimized thermomechanical stresses in the case of using a LTCC balancing element because it has the same coefficient of thermal expansion (CTE) value as the sensor element itself. Another reason comes from the idea that it could be possible to braze the LTCC sensor di-rectly on the Kovar element if the required brazing tempera-ture is low enough to avoid changes in the microstructempera-ture of the complex LTCC sensor. However, in comparison to
alu-mina, less information on brazing of LTCC is available. One approach to braze LTCC is to use a metallized LTCC in com-bination with a non-active braze filler metal. For this purpose Keusseyan and Dilday (1993) investigated the brazeability of Cu-, Ag-, and Au-based thick film metallization layers and concluded that for LTCC–metal joints the brazing temper-ature should be limited to 500◦C in order to minimize the mechanical stresses caused by the mismatching thermal ex-pansion coefficients. Another approach, followed by Walker et al. (2006), was the investigation of PVD thin-film coat-ings like Ti–Au, Ti–Pt, and others. While brazing with a Ag– Cu–In braze filler metal, the Ti–Pt thin films yielded the best hermeticity performance and highest strength. However, on active metal brazing of LTCC without further metallization layers only one study was found in the literature. Further-more Walker et al. (2006) stated that hermetic joints of LTCC and Kovar were only be possible if the LTCC was ground and re-fired prior to the brazing process. As shown, brazing of LTCC only is less investigated, and no systematic results are given in the literature which let one judge about the suit-ability of the active metal brazing technique for LTCC. In the case of Al2O3, besides Ag–Cu–Ti active braze filler
met-als, no brazing alloys with lower brazing temperatures were tested. Thus in the present study the commercially available active braze filler metal Incusil®-ABA is investigated to pro-vide brazing parameters as a means to obtain hermetically brazed Kovar–Al2O3and Kovar–LTCC joints. For
compari-son with the literature, joining of Al2O3and LTCC to Kovar
with Cusil-ABA®was investigated as well.
2 Methods and materials
2.1 Ceramic materials
The LTCC sensors are based on DuPont’s GreenTape 951 system, as this material system is a fair compromise in comparison to other ceramic co-firing materials regarding Young’s modulus and fracture strength. While the exact com-position of this LTCC quality is not published by DuPont, within this work it is important to know that the main crys-talline phase consists of Al2O3grains which are bonded by
a PbO-based glass frit. For joining experiments LTCC sam-ples were made by laminating three tape layers followed by a firing process with a peak temperature of 850◦C similar to the one described by Fournier et al. (2010). After firing the sintered LTCC had a thickness of 630 µm and was cut into single samples (7×7 mm2) with a dicing saw. The sur-face roughness of the as-fired LTCC was Ra < 0.36 µm. The alumina ceramic was obtained in thick film standard qual-ity with an Al2O3 content of 96 % (Rubalit® 708 S,
Table 1.Active metal brazing filler metals with their compositions and brazing temperatures.
Brazing filler metal Ag (wt%) Cu (wt%) In (wt%) Ti (wt%) Brazing temperature
Incusil-25-ABA 43.6 29.1 24.3 3.0 650◦C
Incusil-ABA 59.0 27.25 12.5 1.25 755◦C
Cusil-ABA 63.0 35.25 1.75 810–850◦C
sensor setup indicated in Fig. 1. The bottom side of the ele-ment will be brazed to the steel interconnect and the cavity in the upper side a ceramic balancing substrate made of Al2O3
or LTCC will be brazed by an active metal brazing process. The Kovar components additionally contain a channel struc-ture that allows access of the pressurized gas to the sensor membrane. The surfaces of all samples were degreased prior to assembly and brazing processes.
2.2 Brazing and glass sealing
In accordance with the previously described integration strat-egy, two brazing processes – one for joining the steel con-nect to Kovar and one for joining Kovar to Al2O3 – have
been selected. A one-step brazing process appears to be more favourable but offers fewer opportunities for a process con-trol regarding the hermeticity of the different interfaces. For this reason the brazing processes were separated. For the first brazing joint between steel and Kovar, a nickel-based braz-ing foil (MBF-20 from Metglas Inc., Conway, SC, USA) is applied which has a liquidus temperature well above the ac-tive filler metal brazes used for the second brazing process. For this purpose the MBF-20 brazing foil was cut by a laser process into shapes matching strictly to the joined side of the components. The foils were placed in between both compo-nents adjusted with an additional load on top of the arrange-ment and brazed in vacuum (<1×10−5mbar) with the fol-lowing brazing cycle: from room temperature at 5 K min−1to 940◦C with a hold for 15 min in order to achieve a homoge-nous furnace temperature and than again with 5 K min−1up
to the brazing peak temperature of 1055◦C with an addi-tional hold time of 15 min. Cooling down to room temper-ature was conducted at 3 K min−1.
For joining of the ceramics (Al2O3and LTCC) to Kovar,
three types of braze filler metals provided by Wesgo Met-als (Hayward, CA, USA) were used, which are listed in Ta-ble 1. These alloys were applied in the form of laser cut foils with a thickness of 50 µm. Brazing was carried out in a full-metal vacuum furnace with molybdenum heating el-ements at a pressure <1×10−5mbar. The following pro-cess cycle was used for brazing: heating from room temper-ature to 550◦C (for Incusil-ABA) or 700◦C (Cusil-ABA) at a rate of 10 K min−1, holding for 20 min to obtain a temper-ature homogenization inside the furnace, further heating up to the desired brazing temperature (Table 1), holding of braz-ing temperature for 10 min, and then coolbraz-ing down to room
Figure 3.High temperature pressure measurement station for char-acterizing sensors up to 200 bar and 650◦C.
temperature at a rate of 5 K min−1to 400◦C with subsequent furnace cooling. The brazing temperatures were varied be-tween 810 and 850◦C for Cusil-ABA in order to investigate the influence of brazing temperature on gas-tightness and mi-crostructure of the joined assemblies. For the final integra-tion of the LTTC sensor into this prepared steel connector, a commercially available, lead-free sealing glass from ASAHI (4115DS-NY01) supplied as a ready-to-use paste with an ap-propriate firing profile having a peak temperature of 500◦C for use in muffle furnace was screen-printed on the back side of the LTCC sensor element. For the joining process the sen-sor was placed on the ceramic balancing element together with a mechanical load of 10 g.
2.3 Characterization
Figure 4.Scanning electron micrographs of steel (1.4542)–Kovar joints brazed with MBF-20 showing an overview in(a)and details of the microstructure in(b).
is equipped with an energy dispersive X-ray analysis sys-tem (abbreviation: EDX; Inca x-sight, Oxford Instruments, Abingdon, England), which allows for a quantitative detec-tion of elements.
The characteristic of the pressure sensor is performed by a newly developed pressure measurement system. The sensors can be measured in a special chamber oven KU70/07-A of the company THERMCONCEPT using a high-temperature-capable pressure rail and a ceramic-insulated electrical wiring for a temperature range of 25–650◦C. At the same time, the sensors were applied by means of pressure con-troller PACE 5000 of the company GE Measurement & Con-trol in the range of 0–200 bar with and the characteristic curve is measured with a computer-controlled system. Fig-ure 3 shows the inner setup of this newly developed measFig-ure- measure-ment device which allows the simultaneous characterization of 6 sensors at maximum in the range of 25–600◦C.
3 Results and discussion
As claimed in the Introduction and with respect to the chosen integration strategy, especially the joining process of metals to ceramics represent a challenge. Active metal brazing of Kovar and similar alloys to Al2O3by active metal brazes is
not generally a new topic, but in terms of reduced brazing processing temperatures the use of indium-containing active metal brazes appears attractive. In the case of the SiO2- and
PbO-containing LTCC material it can be supposed that the active component titanium in the active metal brazing alloy will undergo a redox reaction with these oxides. Possible re-action products could be titanium silicides of titanium–lead intermetallic compounds. Especially the formation of sili-cides with different components of high-temperature-stable brazing alloys is described by McDermid and Drew (1991) or Liu et al. (2009). Such intermetallic phases have a brittle character and can have disadvantageous effects on the adhe-sion of the braze at the ceramic surfaces. For these reasons special interest must be paid to the interfacial reactions be-tween the different brazing alloys and joint materials in order to identify proper brazing alloys and brazing conditions.
3.1 Brazing of Kovar and steel
Brazing of the balancing Kovar element into the steel hous-ing is the first step of the integration procedure. In accor-dance with brazing temperatures which are required for ac-tive filler braze between 750 and 850◦C, it is necessary to perform this brazing step at a higher temperature which lies well above the formerly mentioned one. One must take care on stability of the Kovar alloy and potential reactions with the filler braze. Based on these boundary conditions MBF-20, an amorphous nickel braze filler metal was chosen. The brazing process, was performed at temperatures between 1040 and 1060◦C. The micrograph in Fig. 4a shows an overview of the brazing zone indicating a good and pore-free adhesion of both components. A closer look at Fig. 4b reveals the forma-tion of darker chromium borides which are brittle intermetal-lic phases in the brazing alloy. This indicates that chromium from the steel slightly dissolves into the molten brazing al-loy MBF-20, which also contains small amounts of boride for reduced melting temperatures. Without going into much into detail we can say that it was possible to achieve hermetic dense joints with this materials and the SEM investigations gave no hints for significant interfacial reactions. With these results the CTE adjusted steel connector for the further inte-gration of the ceramic components is available.
3.2 Brazing KOVAR to ceramic interlayer
3.2.1 Brazing with Incusil-ABA
Active metal brazing of LTCC and Al2O3 to Kovar with
Incusil-ABA at 755◦C for 10 min yields in both cases to her-metically sealed assemblies. Surprisingly, we were able to realize hermetic joints of as-fired LTCC and Kovar, which is in sharp contrast to the results of Walker et al. (2006). SEM images of the microstructures of Kovar–Al2O3 and
reac-Figure 5.SEM images of Kovar–Al2O3(a)and Kovar–LTCC(b)joints brazed with Incusil-ABA at 755◦C for 10 min.
Figure 6.Enlarged SEM images of Fig. 5a showing the Al2O3/Incusil-ABA(a)and the Kovar/Incusil-12.5-ABA interfaces(b).
Figure 7.Enlarged SEM images of Fig. 5b showing the LTCC/Incusil-ABA(a)and the Kovar/Incusil-12.5-ABA interfaces(b).
tion layers on both interfaces. Figure 6a and b are the en-larged images from Fig. 5a displaying the reaction layers at the interface between Al2O3and the brazing alloy, and
be-tween Kovar and the brazing alloy, respectively. At the inter-face between Al2O3and the brazing alloy a very thin
tion layer with submicron thickness was formed. This reac-tion layer completely covers the alumina interface, yielding a mean helium lake rate of 6×10−10mbar s−1. The main con-stituents of the reaction layer are Ti and O, but also elements of the Kovar, i.e., Ni, Fe, and Co, are detected. This suggests that the constituents of the Fe–Ni–Co alloy show a strong affinity to Ti even at lower temperatures than in the publi-cations of Stephens et al. (2000) and Vianco et al. (2003a). The strong reactivity of Fe, Ni, and Co with Ti shaped the in-terface between Kovar and the brazing alloy as several inter-metallic compounds like (Fe,Ni,Co)xTiywith a high amount of Ni (abbreviation: Ni–Co–Fe–Ti) or Fe (abbreviation: Fe– Ni–Co–Ti) are observed. These intermetallic phases form a
small band which meanders parallel to the Kovar surface. Further away from the interface in the brazing alloy Ni-Cu-Ti compounds are visible. In addition, down to a depth of 25 µm, Ag, In, and Cu from the brazing alloy are found at the grain boundaries of the Kovar and along Fe–Co grains which are depleted of Ni. In comparison with Kovar–Al2O3
joints, the microstructure of Kovar–LTCC joints with Incusil-ABA looks similar. Figure 7a and b are the enlarged images from Fig. 5b showing the interfaces between LTCC and the brazing alloy, and between Kovar and the brazing alloy, re-spectively. Again, through diffusion of Fe, Ni, and Co and their reaction with the active element Ti, intermetallic com-pounds were formed in the brazing alloy and along the inter-face to Kovar. However, in the micrographs two differences in comparison with Kovar–Al2O3 joints are found. Firstly,
Figure 8.SEM images of Kovar–Al2O3joints brazed with Incusil-25-ABA at 650◦C for 10 min.
Table 2.Hermeticity after brazing with Cusil-ABA as a function of brazing parameters.
Joint setup Peak process Hermetic joints temperature (fraction)
Al2O3–Kovar 810◦C 2/6
Al2O3–Kovar 830◦C 12/12
Al2O3–Kovar 850◦C 6/6
LTCC–Kovar 810◦C 2/6
LTCC–Kovar 830◦C 6/6
LTCC–Kovar 850◦C Not tested
the reaction layer is non-continuous with some pores where the brazing alloy was not able to wet the LTCC completely. However, all brazed assemblies were hermetic with an aver-age helium lake rate of 4×10−9mbar s−1.
Further experiments were conducted by using the Incusil-25-ABA brazing paste. An apparent advantage of the brazing alloy is the lower processing temperature between 640 and 680◦C. Within the scope of this study it was not possible to achieve hermetically brazed joints between Al2O3–Kovar
and LTCC–Kovar while using the Incusil-25-ABA brazing alloy. The wetting of the braze on the Kovar surface was ex-cellent, which in turn led to spreading of the melt out of the brazing gap all over the Kovar surface. As a consequence the brazing joints contained numerous and quite large pores, and a porous microstructure was formed as seen in Fig. 8a. Also the reason for the excellent wetting can be taken form the SEM images in Fig. 8. A strong interaction between the ac-tive metal braze and the Kovar alloy leads to the destruction of the microstructure beneath the Kovar surface and is quite more pronounced than is the case for the Incusil-ABA braze. This strong reactivity enables the wetting of the molten braz-ing alloy on the Kovar. Also here the dissolution of the Ko-var into the brazing melt results in the formation of nickel– titanium-based phases in the brazing alloy, which can be rec-ognized as the darker disperse phase in the brazing zone. In accordance with the enhanced dissolution of the Kovar, this phase formation seems to be more pronounced. However Incusil-25-ABA contains more titanium than Incusil-ABA,
Figure 9.SEM image of a Kovar–LTCC joint brazed with Cusil-ABA at 810◦C for 10 min.
which may also be a reason for the stronger phase forma-tion. This reaction captures at least a fraction of the active-phase titanium from the brazing alloy which is necessary to enable a wetting process on ceramic surfaces. Thus due to insufficient brazing results no further experiments were per-formed with this brazing alloy containing a high percentage of indium. The presented results showed that brazing of Ko-var with indium-containing active braze filler metals leads to considerable destruction of the Kovar microstructure. An op-timization of the brazing cycle could help to minimize this behaviour. However this was beyond the scope of the present study and will be addressed in the future. As explained in the next section, the indium-free Cusil-ABA braze filler metal leaves the microstructure of the Kovar nearly intact.
3.2.2 Brazing with Cusil-ABA
Active metal brazing of Al2O3 and LTCC to Kovar with
Cusil-ABA was performed at three different brazing temper-atures for a minimum of six samples for each brazing con-dition. Table 2 summarizes the obtained hermeticity data. While brazing of Al2O3 yielded to hermetically sealed
Figure 10.SEM images of Kovar–Al2O3(a)and Kovar–LTCC(b)joints brazed with Cusil-ABA at 830◦C for 10 min.
Figure 11.Enlarged SEM images of Fig. 10a showing the Al2O3–Cusil-ABA(a)and the Kovar–Cusil-ABA interfaces(b).
because it seems that only a non-continuous and thin reaction layer was formed. Thus at this brazing temperature no reli-able joining was possible. When the brazing temperature was increased to 830◦C, all brazed LTCC–Kovar joints showed gas-tightness due to the formation of a continuous reaction layer at the interface between LTCC and Kovar, which is shown later in detail. Due to the fact that this brazing tem-perature is close to the sintering temtem-perature of the LTCC tape, it was initially assumed that brazing is not possible be-cause of softening of the residual glassy phase in the LTCC. This was not confirmed, and the results showed strong and hermetic bonding. However, in contrast to Al2O3, brazing of
LTCC at 850◦C was not tried as the LTCC is sintered at this temperature and the stability of the ceramic material is lim-ited. In fact a repeated heating of the DuPont 951 tape up to the processing temperature is possible without any degrada-tion of the microstructure. Scanning electron micrographs of the microstructures of Kovar–Al2O3and Kovar–LTCC joints
brazed with Cusil-ABA at 830◦C are shown in Fig. 10a and b, respectively. In these micrographs the brazing alloy dis-plays the typically structure of the Ag–Cu eutectic with a Ag-rich phase (white region) and a Cu-rich phase (grey re-gions). Furthermore, the formation of reaction layers on both interfaces is visible.
These reaction layers are shown in more detail in Fig. 11a and b. The active element titanium formed a continuous re-action layer with a thickness of 0.7–1 µm bordering the inter-face between Al2O3and the brazing alloy. The reaction layer
consists of titanium and oxygen with minor amounts of Ni, Fe, and Co. The elemental composition is the same as ob-served for Kovar–Al2O3joints brazed with Incusil-ABA. At
the interface between Kovar and the brazing alloy an up to 3 µm thick reaction layer with multiple phases was formed.
The main phase comprises a Fe-rich intermetallic compound (Fe–Ni–Co–Ti) that covers the interface of the Kovar com-pletely. Adjacent to the Fe–Ni–Co–Ti phase a second Ni-rich phase (Ni–Co–Fe–Ti) was found. A third intermetallic phase composed of Ni, Cu, and Ti is observed in the brazing alloy. The microstructural analysis of Kovar–Al2O3 joints
Figure 12.Enlarged SEM images of Fig. 10b showing the LTCC–Cusil-ABA(a)and the Kovar–Cusil-ABA interfaces(b).
Table 3. Compositions of interfacial reaction layers Cusil-ABA– LTCC at 830◦C for 10 min and Incusil-ABA–LTCC after brazing at 755◦C for 10 min.
Element Cusil-ABA– Incusil-ABA–
Ma. % LTCC LTCC
O 10.2 16.5
Al 0.6 1.6
Si 3.8 4.5
Ti 44.8 36.4
Fe 6.7 10.1
Co – 3.9
Ni 8.2 10.1
Cu 25.8 13.0
In – –
Ag – –
note that a more detailed investigation of the grain boundary phase is necessary in order to clarify this behaviour.
The microstructure of Kovar–LTCC joints brazed with Cusil-ABA is similar to the microstructure of Kovar–Al2O3
joints. Figure 12a and b are the enlarged images from Fig. 10b displaying the reaction layers at the interface be-tween LTCC and the brazing alloy, and bebe-tween Kovar and the brazing alloy, respectively. At the interface between LTCC and the brazing alloy a nearly 1 µm thick reaction layer was formed (Fig. 12a). Besides the main constituents of tita-nium and oxygen, the EDX analysis revealed the presence of Fe, Ni, and Co from Kovar and of minor traces of silicon and lead from the glass phase of the LTCC. It is noteworthy that the compositions of the interfacial reaction layers bordering the LTCC interface are quite similar to the one found after brazing of LTCC and Kovar with Incusil-ABA as seen by EDX data in Table 3, which compares the compositions of the interfacial layers brazed with Cusil-ABA at 830◦C and with Incusil-ABA at 755◦C. A large difference is only rec-ognized for the copper content. However we should not for-get the small thickness of the reaction layer in the case of Incusil-ABA which adds an uncertainty to the spectral data. So additionally in Table 4 similar results are presented for compositions of two interfacial layers resulting from braz-ing both LTCC and Al2O3 with Cusil-ABA at 850◦C for
10 min. However, the higher brazing temperature for Cusil-ABA yielded a much thicker reaction layer than for samples
Table 4.Compositions of interfacial reaction layers Cusil-ABA– LTCC and Cusil-ABA–Al2O3after brazing at 850◦C for 10 min.
Element Cusil-ABA– Cusil-ABA–
Ma. % LTCC Al2O3
O 14.3 12.5
Al 1.2 4.14
Si 4.7
Ti 32.9 36.6
Fe 11.6 12.6
Co 5.6 4.7
Ni 24.0 25.0
Cu 4.5 4.3
Ag 1.0 1.3
brazed with Incusil-ABA. The comparison of the interface between Kovar and the brazing alloy after brazing to Al2O3
(Fig. 11b) and LTCC (Fig. 12b) showed no difference in mi-crostructural appearance like thickness, phases formed, or el-emental composition. Based on these results it was decided to use the Cusil-ABA braze filler metal with a brazing tempera-ture of 830◦C for the construction of the complete sensor as shown in the next section.
3.2.3 Joining of sensor and electrical connection
In accordance with the integration procedure the last step in-volves the soldering of the sensor element by a glass paste which was screen-printed on the back side on the sensor and fired at maximum temperature of 550◦C. Joining and sealing processes for packaging of ceramic-based sensor elements are established for quite a long time, and so numerous quali-fied glass solders are available for this task. Figure 13a illus-trates the final assembling steps of the sensor element into the steel connector with both brazed balancing elements made of Kovar and alumina. In Fig. 13b a SEM image shows the joining zones Kovar–Al2O3 and Al2O3–LTCC sensor of a
Figure 13.Joint components showing the stepwise integration procedure(a)and a SEM image of a cross section of a packaged LTCC pressure sensor(b).
Figure 14.Temperature-dependent pressure-signal characteristics of a completely packaged LTCC sensor.
a completely assembled sensor based on an applied pres-sure (bar) and the corresponding sensor signal (1mV/V) between 25 and 300◦C. The sensor signal shows a good lin-earity in the investigated pressure and temperature range. The sensitivity remains unaffected, and the particular curves are only shifted by a small offset, which can be compensated by an accompanied temperature measurement.
4 Conclusions
The present work focused on the joining process of Ko-var with alumina and LTCC as part of on approach to in-tegrate LTCC-based sensors into steel connects. While us-ing commercially available active braze filler metals (Cusil-ABA, Incusil-(Cusil-ABA, Incusil-25-ABA) under certain condi-tions, both ceramic types were hermetically sealed to Ko-var. Hermetic joining of Al2O3 to Kovar was possible with
Incusil-ABA and with Cusil-ABA for all investigated tem-peratures. Additionally, brazing of LTCC to Kovar was possi-ble and shown for the first time. At 755◦C with Incusil-ABA hermetic LTCC–Kovar joints were realized. The higher in-dium content of Incusil-25-ABA would enable lower brazing
Figure 15.Completely assembled sensor with steel connect screw and welded steel housing with wiring.
Acknowledgements. The authors thank Felix Köhler, Birgit Manhica, Maria Striegler, and Sabine Fischer for sample prepa-ration, helium leak rate measurement, and scanning electron microscopy.
Edited by: H. Fritze
Reviewed by: two anonymous referees
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